(1) Field of the Invention
This invention relates generally to motor control systems and more specifically to methods and devices for starting and controlling a motor having at least two speeds.
(2) Description of the Prior Art
In various applications of capacitor-start motors, it is necessary to provide multiple speed operation. For example, a washing machine may be required to provide operation at a pair of speeds, such as a first speed for general loads, and a second, slower speed for delicate loads. A typical application employs a two-speed motor and provides a selectable start speed, i.e., the motor can be started for operation at one of a pair of speeds, and the motor runs at that speed until it is stopped. Usually, one of the speeds is used more frequently than the other. Thus, a switching device that provides high reliability when used to start the motor at the more frequently used speed, and that provides an economical way to accommodate a lesser number of starts at the less frequently used speed could advantageously be used in this application.
A dual-speed motor for use in this type of application typically has a first run winding having a first number of poles to provide a first motor speed, a separate start winding having the same number of poles to start the motor, and a second run winding having a second, different number of poles to provide a second motor speed. The motor is started by energizing the first run winding and the separate start winding. As the motor increases its speed, the start winding is typically switched off and, based upon the selected speed of the motor, either the first run winding continues to be energized, or else the first run winding is deenergized and the second run winding is energized. Usually, the number of poles of the second winding is greater than the number of poles of the first winding, making the motor's second speed lower than its first speed. A motor of this type can be used with a dual-speed switching arrangement to provide multiple speed operation of an electrical appliance, such as the washing machine mentioned above.
Traditionally, a mechanical centrifugal actuator switch has been used to drop out the start winding of a single-speed motor upon reaching run speed. However, a centrifugal actuator used in this manner functions every time the motor is started. The relatively limited lifetime of the contacts and mechanisms of this type of mechanical switch (typically around 100,000 to 300,000 or fewer operations), as well as the fact that the centrifugal actuator must be mounted on the rotor of a motor, makes the use of a centrifugal actuator switch undesirable for some applications. Adapting such a switch for two speed operation, while possible, is not preferable where high reliability is required for at least one speed.
One example of a prior art motor with a dual-speed switching arrangement is described in U.S. Pat. No. 4,443,749 to Douthart et al. The control circuit described therein has one power terminal connected to a two pole main winding of a motor, and another power terminal connected to a four pole main winding. The four pole main winding is connected to the power terminal through a solid state gated switch, which is normally open. One of the power terminals of the control circuit is connected to a power supply line, depending upon the desired operating mode. In either operating mode, power is also supplied through a triac to a start winding, causing the motor to start.
When power is applied to the terminal representing the two pole operating mode, a solid state switch prevents power from being applied to the four pole winding, and the motor starts as a two-pole motor. When a conventional mechanical centrifugal switch attached to the rotor opens, power is removed from the start winding and the motor continues to run as a two-pole motor, because the two pole run winding remains energized. On the other hand, when power is applied to the terminal representing the four pole operating mode, a solid state switch once again prevents power from being applied to the four pole winding, so the motor starts as a 2-pole motor. However, when the centrifugal switch opens in this operating mode, power is removed from the two pole run winding and the main power path of the triac in the start winding circuit, simultaneously disconnecting both the two pole run winding and the start winding. Power is then applied to the solid state switch associated with the four pole winding, so that the switch turns on and supplies power to the four pole winding. The motor then operates as a four pole motor.
The circuit of Douthart et al. '749 provides a motor that can be started at two different speeds, but requires a conventional mechanical centrifugal switch to accomplish this purpose. As noted above, these switches require placement of actuator mechanisms on the rotor of the motor, and thus, it may be physically inconvenient to supply the necessary space for this switch. Also as noted above, mechanical centrifugal switches have relatively limited operational lifetimes. In the Douthart et al. '749 circuit, the latter problem may be exacerbated because the centrifugal switch contacts always switch power when the motor is operated, irrespective of the selected mode of operation. Arcing during switching operations tend to wear down switch contacts, or may cause them to weld together. It would therefore be desirable to avoid the use of mechanical centrifugal switches altogether, or at least to avoid the frequent switching of power through electromechanical contacts.
Related U.S. Pat. No. 4,467,257 to Douthart et al. describes a circuit in which a two-pole run winding, a four-pole run winding, and a start winding are provided. Power is supplied by one of two power terminals, depending upon the desired motor operation mode. As in Douthart et al. '749, a conventional mechanical centrifugal switch is used to switch between the speeds after start-up. The centrifugal switch switches power only during the four-pole (i.e., the slower) operating mode, which provides some improvement in reliability over the circuit arrangement of Douthart et al. '749, but the centrifugal switch actuator still must be inconveniently mounted on the rotor, and reliability could be further improved through total elimination of the centrifugal switch. Another disadvantage of this circuit is that it requires two high-current solid state switches and a transformer, the latter being required to operate a gate of one of the solid state switches. It would be advantageous to reduce the total number of high current solid state switches to reduce heat sinking and isolation requirements.
U.S. Pat. No. 5,514,943 to Shapess describes a motor control system having a first speed circuit and a second speed circuit. Both circuits are connected to a switching device that controls the flow of current to each circuit. The first speed circuit includes a first speed primary winding, a first speed secondary winding, a start capacitor, and a start relay. The second speed circuit includes a second speed winding. The motor control system initially engages the first speed circuit regardless of which speed of operation is selected. The switching device closes its contacts to excite the first speed circuit, and a start relay is closed for a predetermined amount of time to excite a start circuit that provides power to the secondary winding. In high speed operation, the switching device keeps the first speed circuit active, while holding the second speed circuit inactive. In low speed operation, the switching device deactivates the first speed circuit after the motor starts, while activating the second speed circuit. No specific types of switches are disclosed in this reference for the switching device, which is described only as any device that may be capable of manually, mechanically, or electrically opening or closing an electrical circuit. Shapess '943 does not teach or suggest solutions to the issues of reliability of the switching devices, how such devices might automatically operate at appropriate times during the start of the motor, or how a device that controls current in a start winding might cooperate with a switch to control current through the main windings after the motor has started.
U.S. Pat. No. 4,030,008 to Buckle et al. describes a motor having a high speed main run winding, a start winding, and a low speed main winding. A selector switch, in conjunction with an on/off power switch, provides for the selection of high speed or low speed operation. For high speed operation, the selector switch and power switch are set to complete a circuit through the high speed main winding. Current through the high speed winding energizes a first relay coil, causing the relay's contacts to close, which energizes the start winding and starts the motor. As the speed of the motor increases, current through the high speed winding decreases until the relay drops out (i.e., the contacts open), which interrupts the flow of current through the start winding. For low speed operation, the switches are set to first energize the low speed winding through a triac. (The stated function of the triac is to inhibit the flow of current in the low speed main winding when the motor is suddenly switched to low speed operation from high speed operation). The current flowing through the low speed winding causes a second relay's contacts to close, which energize the high speed main winding, which, in turn, causes the first relay's contacts to close, which energizes the high speed start winding. The energizing of the start winding causes the motor to start, which decreases the flow of current in the low speed winding to a point at which the second relay drops out, which shuts off the current flow to both the high speed main winding and the start winding.
The starter circuit described in Buckle et al. is thus fully controlled by electromechanical relays, the triac being used only to inhibit the flow of current in the low speed main winding when the motor is suddenly switched to low speed. Thus, it is necessary to switch power through at least one relay irrespective of the mode in which the motor is started, making the reliability of the starter circuit essentially equal to the reliability of the powered relay contacts. As discussed above, these contacts are subject to damage from arcing and even possible welding with each powered switch cycle. Furthermore, all three windings in the Buckle et al. circuit are energized during a motor start, which undesirably increases the current requirement of the motor as it comes up to speed.
The disclosure of U.S. Pat. No. 4,030,009 to Halsted '009 is essentially similar to that of Buckle et al. '008, except that the former does not disclose any circuitry to inhibit the flow of current in the low speed main winding when the motor is suddenly switched to low speed from high speed operation.
While dual speed capacitor-start motors and circuitry are shown in the above noted prior art references, they do not teach or suggest a switching device in the starting circuitry with the desirable feature of being able to accommodate a large number of starts at a first speed with high reliability, while economically accommodating a lesser number of starts at a second speed, without substantially reducing the reliability of starts at the first speed. It would also be desirable for such a switching device to avoid the use of mechanical centrifugal switches, which have relatively low reliability and which must have an actuator mounted on the moving part of the machine. It would additionally be desirable for such a switch to not require more than one power switching solid state device to thereby avoid excessive heat sinking requirements or complex isolation circuitry to operate, and for the switch to provide automatic switching between modes of operation during start-up when required without unnecessarily increasing the starting current required by the motor.